this is how I would hook this up. Run your 480v feeders to H1 and H4. Then run a jumper wire between H2 and H3. Next in order to get 120v, you would jump x1 and x2 - then jump x3 to x4. Dont hook anything up to the secondary. Just power the unit up and check your voltages on the secondary to be sure your getting 120.

In that case you would take a voltage reading from each terminal to ground to see which one does not register voltage.

Now I must caution you here, my answers to you on this post are based on the fact that most transformers like yours would be wired this way. Without having the manufacturers numbers on the unit, or the units diagram I cannot answer you with 100% certainty. I say this just so you are aware that just because I say it, does not mean it will be true in your case. That is why you must always take voltage readings before hooking any kind of a load up to this.